1. Field of the Invention
This invention relates to brazing filler metals composed of nickel-chromium-based alloys containing transition metals such as iron and molybdenum and certain metalloids; and more particularly to multicomponent alloys containing nickel, chromium, iron, molybdenum, boron, and silicon, which are particularly useful for brazing metals at high temperatures to produce high strength and corrosion-resistant brazements
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a brazing filler metal that has a melting point lower than that of the parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong, preferably corrosion resistant, joint is formed.
One class of products produced by brazing processes is plate-type heat exchangers. The plate-and-frame heat exchangers have been widely used in food, chemical, aerospace and other process industries. A standard plate-and-frame heat exchanger consists of a number of alternating corrugated/flat metal sheets kept in tight, sealed contact with each other using gaskets or being brazed. In general, brazed heat exchangers are stronger and more suitable for high temperature/high pressure applications than those with the gasket-type of sealing. These plates are mounted on a frame that may be free-standing or are built into a supporting structure. An elaborate system of channels is formed by these plates in which two, one hot and one cool, liquid and/or gas media flow separately exchanging heat and thus saving energy. Fully brazed units have been used for some time with mostly noncorrosive mediums, such as CFC, etc. Therefore, the brazing has been carried out using stainless steel as a base metal and mostly copper as a filler metal. Recently, because of the general gradual withdrawal of CFC as a refrigerant and replacement of it with preferably ammonia and also the improving of energy savings in some new application area, there is a great need for development of noncorrosive stainless steel brazes which can withstand corrosive effects of ammonia and, at the same time, some other even more potent than ammonia mediums including sea water, various acid solution, etc. Such alloys should be compatible with stainless steel base metals and, therefore, nickel-chromium-based filler metals are the best potential choice for such an application. Moreover, of particular importance for heat exchanger performance is the ability of their brazed component to resist fatigue appearing due to alternating thermal stresses. Therefore, heat exchanger brazes should be free from any brittle phases in order to provide high ductility and strength combined with high corrosion resistance. Nickel-chromium-based filler metals containing boron and silicon, when crystallizing from the liquid state, form the eutectic mixture of solid solution and intermetallic solid phases. It is well known that the presence of intermetallic phases rich in boron and silicon results in formation of brazes which are brittle, poorly resistant to fatigue, and susceptible to corrosion. Conventional brazing is typically carried out with a holding time at brazing temperature of about 15-30 min. Such holding time is insufficient to deplete the braze of silicon and boron by dissolving these components into adjoining base metal parts of sufficient thickness to a degree that the intermetallic phases cannot be formed. The most natural way to accomplish such depletion of brazes is to increase the brazing temperature and to extend the holding time in a high temperature range during the brazing operation. At the same time, it should be kept in mind that the dissolution of silicon and boron in base metal components should not result in the base metal brittleness due to formation of intermetallic phases in the base metal itself.
Brazing filler metals consisting of nickel-chromium-based alloys have been developed which exhibit high temperature strength and good corrosion resistance. Such alloys have been disclosed, for example, in U.S. Pat. Nos. 4,184,973, 4,302,515, 4,745,037, and 4,543,135. The alloys disclosed in these patents, however, each exhibit drawbacks which make them unsuitable for brazing products which require prolonged service life in highly corrosive environments or having a sufficient thickness to provide sufficient braze cross-section with high effective joint strength. For example, the alloy species disclosed in U.S. Pat. Nos. 4,148,973, 4,302,515 and 4,745,037 contain boron in substantial amount(s) (at least 9 to 18 atom percent). It is well known that boron diffuses extensively out of the joint area into to stainless steel and superalloy base metals when subjected to brazing at very high temperatures and forms intermetallic boride phases detrimental to base metal mechanical fatigue and corrosion resistance. Specifically, boron, with its small atomic radius, diffuses along grain boundaries forming therein intermetallic borides and resulting in brittle fracture under loading at elevated temperatures. Therefore, alloys containing a large amount of boron are not suitable for brazing products designed to withstand high temperature, high stress and high fatigue environments, i.e., for example, stainless steel and/or superalloy honeycomb structures employed in airfoils and plate-type heat exchangers subjected to variable high pressure/high temperature conditions. Moreover, this is of the critical importance for structures in which the thin gage (about 0.1-0.05 mm thick) base metal components are used.
Regarding the quaternary nickel-chromium-silicon-boron alloys disclosed in U.S. Pat. No. 4,543,135, these alloys have large concentrations of nickel and chromium, moderate concentration of silicon and small concentration of boron. Although the problem associated with boron is mostly avoided, the high chromium content results poor ribbon ductility. Specifically, these alloys cannot be produced as a wide, ductile foil having thickness of about or greater than 25 .mu.m. It is well known from the existing art that chromium, when compared with such elements as nickel, palladium, and iron, decreases amorphability of alloys containing transition elements and metalloids such as boron and silicon. The low maximum thickness (about 25 .mu.m) and maximum width (.ltoreq.100 mm) at which the 19% chromium containing brazing foil retains ductility, causes joints produced from this ribbon to be thin and weak. Moreover, the alloys disclosed by U.S. Pat. Nos. 4,148,973, 4,302,515, and 4,543,135 contain no molybdenum. The only molybdenum containing species taught by U.S. Pat. No. 4,745,037 requires the presence of 20 atom percent cobalt, which element is expensive and significantly increases the cost of the foil. Moreover, the molybdenum containing species taught by U.S. Pat. No. 4,745,037 requires 16 atom percent boron, which would further increase the cost, increase errosion of the base metal, and decrease the strength and corrosion resistance of the joint. In direct contradistinction with the teachings of these patents, it has been found that the presence of molybdenum with from 6 to less than 7 atom percent boron, greatly improves corrosion resistance of nickel-chromium-based alloys to solutions of halogen salts and to pitting in seawater. This improved corrosion resistance is of specific importance in heat exchanger applications in which water is used as a cooling medium or when water is preheated before being converted to steam in power plants to save energy. For the above reasons, the alloys taught by prior art workers are not effective for use in brazing products to be employed in high temperature, high stress and high fatigue environments, such as heat exchanger applications.
Accordingly, there remains a need in the art for improved brazing filler materials suitable for brazing stainless steels and superalloys at high temperatures. There remains further a need for an improved brazing process productive of brazements which exhibit optimal microstructure and thickness, and which retain high strength and high corrosion resistance at elevated temperatures over prolonged periods of time.